The recovery or conversion of various mixtures or impure lead compounds and possibly including lead metal to a pure commercial grade of lead oxide (PbO, massicot or litharge) of high purity (99.7+%) presents many unsolved problems. This conversion of lead acid battery pastes (lead sulfate, red lead, Pb3O4, lead sulfate, lead fluoride, lead chloride, lead carboxylate salts, etc.) often requires a reduction of the impure lead compound to the +2 valence state from the +3 (red lead) or the +4 state. However, in some cases, the requirement is also for the oxidation of any lead metal particle contaminates to the +2 state so that the total lead content is in the same oxidation state for achieving high purity of the product. Lead oxide (PbO) is the plus two valence state oxide of lead and is one of the principal commercial lead compounds of commerce. It can come in several versions or phases particularly litharge (orange red color) or massicot (white or pale yellow) and an amorphous white version. Chemically pure PbO (99.7+%) represents an ideal commercial lead compound to convert any mixture or impure lead compound for recovery or for purification purposes.
The largest source of impure or mixed lead compounds requiring recovery or recycling are the lead acid battery pastes which can be a mixture of red lead and lead sulfates and can contain lead dioxide and finely divided lead metal and other lead oxides. In addition, the complete removal of other known metallic impurities such as antimony, barium, iron, and potentially arsenic, tellurium, silver is required. However, other contaminated lead compounds or mixtures of lead compounds can also present a problem for recovery and purification. Usually all of these impure lead compounds, and the lead acid battery pastes are sent to lead smelters where they are mixed with carbon and heated to 800-1000° C. to form lead metal and to remove large amounts of sulfate forming sulfur dioxide and carbon dioxide which must be fluxed and sparged with air to remove impurities such as antimony, arsenic, iron, and barium which are present in lead acid battery pastes as a impure toxic slag which requires disposal as a hazardous material. This is both an energy intensive process and a highly polluting process due to gaseous and particulate emissions. The disposal of the solid wastes from the slags is also a major concern.
Lead acid electrochemical cells which are otherwise known as “lead-acid batteries” are commonly used to store and deliver electrical energy. For example, lead acid electrochemical cells are normally employed in vehicles (e.g. cars, trucks, boats, aircraft, and the like) for ignition, lighting, and other related purposes.
Conventional lead-acid electrochemical cells include electrically-conductive positive and negative current collectors typically manufactured in the form of foraminous (porous) metallic grids with coatings of a paste composition which are manufactured from a lead alloy or elemental lead (99.9%-99.99% purity lead) as noted in U.S. Pat. No. 3,951,688. The positive and negative pastes are typically produced from one or more particulate lead-containing compositions which may consist of, for example, lead (Pb) or lead compounds (e.g. oxides such a PbO, PbO2 and/or Pb3O4 “red lead” and PbSO4. The selected lead-containing compositions are then combined with a paste “vehicle” (e.g. water) and various other optional ingredients including sulfuric acid. Other additives of interest comprise expander materials as discussed in U.S. Pat. No. 4,902,532 which include barium sulfate, carbon black, and lignosulfonate which are primarily used in connection with the negative paste.
The paste composition positioned on the positive current collector to form the positive plate in an electrochemical cell is typically characterized as the “positive paste”, while the paste composition located on the negative current collector to produce the negative plate is known as the “negative paste”. Further information regarding these items and other characteristics of battery paste compositions in general are presented in U.S. Pat. No. 4,648,177 which is incorporated herein by reference. Likewise, methods of applying the paste compositions listed above to the positive and negative current collectors are specifically discussed in U.S. Pat. Nos. 3,894,886; 3,951,688 and 4,050,482 which are also incorporated by reference.
In 1915, in the U.S. Pat. No. 1,148,062, a process is described for the recovery of lead from exhausted batteries. According to this patent, extracted spent battery pastes are transformed into lead oxides by calcinations and desulfurization. However, the oxides produced are not of high purity.
According to U.S. Pat. No. 4,222,769, an extracted spent battery paste is desulfurized and then transformed into metallic lead by roasting in the presence of a carbon reducing agent.
In U.S. Pat. No. 4,769,116, a paste is obtained from exhausted lead-acid batteries and treated with sodium hydroxide to produce a solution of sodium sulfate and a desulfurized paste. Pure metallic lead is further recovered from the desulfurized paste by electrowinning.
U.S. Pat. No. 5,211,818 discloses a process wherein the paste sludge resulting from the exhausted batteries is treated with a solution of ammonium sulfate and the metallic lead constituent is recovered by electrowinning.
International Publication No. WO99/44942 discloses a process of producing lead monoxide from spent lead batteries using fluxing agents and an organic reducer in the calcinations step at a temperature of 400° C.-450° C.
Currently mixed lead oxides recovered from battery lead oxide pastes are mixed with carbon and smelted directly in a furnace to make molten lead. Then the resulting lead in ingot form is shipped to a plant which has a Barton reactor or ball mill process to be oxidized to the leady lead oxide. The material is subsequently oxidized in another furnace to red lead or pure litharge depending upon temperature and other conditions. This prior art procedure is more costly, requires greater energy expenditure, and requires pollution control.